The design of printed circuit board is based on circuit schematic diagram, and realizes the functions required by circuit designers. The design of printed circuit board mainly refers to layout design, and the layout of external connections needs to be considered. Optimal layout of internal electronic components. Optimal layout of metal connecting lines and through holes. Electromagnetic protection. Heat dissipation and other factors.
When designing PCB (printed circuit board), one of the most basic issues to be considered is how many wiring layers, ground planes and power planes are needed to realize the required functions of the circuit. The determination of the number of wiring layers, ground planes and power planes of the printed circuit board is related to the requirements of circuit function, signal integrity, EMI, EMC and manufacturing cost. For most designs, there are many conflicting requirements for PCB performance requirements, target cost, manufacturing technology and system complexity, etc. The laminated design of PCB is usually decided by compromise after considering various factors. High-speed digital circuits and whisker circuits are usually designed with multilayer boards.
Here are eight principles to pay attention to in cascade design:
1. Layering
Multilayer PCB usually contains signal layer (S), power supply (P) plane and ground (GND) plane. The power plane and the ground plane are usually undivided solid planes, which will provide a good low-impedance current return path for the current of adjacent signal traces. Most of the signal layers are located between these power supply or ground reference plane layers, forming symmetric striplines or asymmetric striplines. The top and bottom layers of multi-layer PCB are usually used to place components and a few traces. These signal traces should not be too long to reduce the direct radiation generated by traces.
2. Determine the single power supply reference plane (power supply plane)
Using decoupling capacitor is an important measure to solve the power supply integrity. Decoupling capacitors can only be placed on the top and bottom layers of PCB. The traces, pads and vias of decoupling capacitors will seriously affect the effect of decoupling capacitors, which requires that the traces connected to decoupling capacitors should be as short and wide as possible, and the wires connected to vias should be as short as possible. For example, in a high-speed digital circuit, the decoupling capacitor can be placed on the top layer of PCB, the second layer can be allocated to the high-speed digital circuit (such as processor) as the power layer, the third layer as the signal layer, and the fourth layer as the ground of the high-speed digital circuit.
In addition, try to ensure that the signal traces driven by the same high-speed digital device take the same power supply layer as the reference plane, and this power supply layer is the power supply layer of the high-speed digital device.
3. Determine the multi-power reference plane.
The multi-power reference plane will be divided into several physical areas with different voltages. If the signal layer is close to the multi-power layer, then the signal current on the signal layer nearby will encounter an undesirable return path, resulting in a gap in the return path. For high-speed digital signals, this unreasonable return path design may bring serious problems, so it is required that the wiring of high-speed digital signals should be far away from the multi-power reference plane.
4. Determine multiple ground reference planes (ground planes)
Multiple ground reference planes (ground planes) can provide a good low-impedance current return path, which can reduce the common-mode EMl. The ground plane and the power plane should be closely coupled, and the signal layer should also be closely coupled with the adjacent reference plane. This can be achieved by reducing the dielectric thickness between layers.
5. Reasonably design wiring combination.
The two layers spanned by a signal path are called a “wiring combination”. The best wiring combination design is to avoid the return current flowing from one reference plane to another, but from one point (plane) of one reference plane to another. In order to complete complex wiring, it is inevitable to switch between layers. When switching between signal layers, it is necessary to ensure that the return current can smoothly flow from one reference plane to another. In a design, it is reasonable to use adjacent layers as a wiring combination. If a signal path needs to span multiple layers, it is usually not a reasonable design as a wiring combination, because a path through multiple layers is not smooth for the return current. Although the ground bounce can be reduced by placing decoupling capacitors near vias or reducing the dielectric thickness between reference planes, it is not a good design.
6. Set the wiring direction.
On the same signal layer, it should be ensured that most wiring directions are consistent, and at the same time, it should be orthogonal to the wiring directions of adjacent signal layers. For example, the wiring direction of one signal layer can be set to “Y axis” and the wiring direction of another adjacent signal layer can be set to “X axis”.
7. Adopt even layer structure.
From the designed PCB laminate, it can be found that almost all the classic laminate designs have even layers, not odd ones. This kind of urgency is caused by many factors, as shown below.
From the manufacturing process of printed circuit boards, we can know that all the conductive layers in printed circuit boards are saved on the core layer, and the material of the core layer is usually double-sided cladding. When the core layer is fully utilized, the conductive layers of printed circuit boards will be even.
Even-numbered printed circuit boards have cost advantages. Because there is one layer of dielectric and copper clad less, the cost of raw materials of odd-numbered printed circuit boards is slightly lower than that of even-numbered printed circuit boards. However, the processing cost of odd-numbered printed circuit boards is obviously higher than that of even-numbered printed circuit boards, because the odd-numbered printed circuit boards need to add a non-standard laminated core bonding process on the basis of the core structure process. Compared with the common core structure, adding copper clad outside the core structure will lead to the decrease of production efficiency and the extension of production cycle. Before laminating and bonding, the outer core layer needs additional processing, which increases the risk of scratching and wrong etching of the outer layer. The additional outer layer treatment will greatly increase the manufacturing cost.
When the inner layer and the outer layer of the printed circuit board are cooled after the multi-layer circuit bonding process, different lamination tensions will cause the printed circuit board to bend in different degrees. Moreover, with the increase of the thickness of the circuit board, the risk of bending of the composite printed circuit board with two different structures is greater. The odd-numbered printed circuit board is easy to bend, and the even-numbered printed circuit board can avoid the bending of the circuit board.
When designing, if there is an odd number of layers stacked, the following method can be used to increase the number of layers.
If the power layer of printed circuit board is designed to be even and the signal layer is odd, the method of adding signal layer can be adopted. The addition of signal layer will not lead to the increase of cost, but can shorten the processing time and improve the quality of printed circuit board.
If the power layer of the printed circuit board is designed to be odd and the signal layer is even, the method of increasing the power layer can be adopted. Another simple method is to add a ground layer in the middle of the stack without changing other settings, that is, first wiring the odd-numbered printed circuit boards, and then copying a ground layer in the middle.
In microwave circuits and mixed dielectric (dielectric with different dielectric constants) circuits, a blank signal layer can be added near the center of the printed circuit board stack, which can minimize the stack imbalance.
8. Cost considerations
In terms of manufacturing cost, with the same PCB area, the cost of multilayer circuit boards is definitely higher than that of single-layer and double-layer circuit boards, and the more layers, the higher the cost. However, when considering the realization of circuit functions and miniaturization of circuit boards, and ensuring signal integrity, EMl, EMC and other performance indicators, multilayer circuit boards should be used as much as possible. Comprehensive evaluation, the cost difference between multilayer circuit board and single and double-layer circuit board is not much higher than expected.